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comp.lang.c++

Trouble with vector::iterator

Prasad

11/19/2008 12:17:00 AM

Hi,

I have been using vector::iterators for a while now. This is the first
time I have encountered this problem.
The vector contains one element.

1. vector<GroupSetTemplate>::iterator gstIt;
2. for(gstIt= this->invariantState.getGroupSetTemplates().begin();
gstIt!=this->invariantState.getGroupSetTemplates().end();gstIt++)
3{
4 cout<<"Gst name"<<(*gstIt).getName()<<endl;
5}

Line 4 throws me segmentation fault.

However if I rewrite the above code using vector::at() it works fine.
GroupSetTemplate element = invariantState.getGroupSetTemplates().at
(0);
cout<<"Gst name"<<element.getName()<<endl;

So I know that there is no problem with the element. Am I missing
something ?
10 Answers

Andrey Tarasevich

11/19/2008 12:26:00 AM

0

Prasad wrote:
> I have been using vector::iterators for a while now. This is the first
> time I have encountered this problem.
> The vector contains one element.
>
> 1. vector<GroupSetTemplate>::iterator gstIt;
> 2. for(gstIt= this->invariantState.getGroupSetTemplates().begin();
> gstIt!=this->invariantState.getGroupSetTemplates().end();gstIt++)
> 3{
> 4 cout<<"Gst name"<<(*gstIt).getName()<<endl;
> 5}
>
> Line 4 throws me segmentation fault.
>
> However if I rewrite the above code using vector::at() it works fine.
> GroupSetTemplate element = invariantState.getGroupSetTemplates().at
> (0);
> cout<<"Gst name"<<element.getName()<<endl;
>
> So I know that there is no problem with the element. Am I missing
> something ?

No, you don't know if there's a problem with the element. In your second
version (with 'at') you create a _copy_ of the vector element, while in
the first version you access the vector element itself. The process of
creating a copy can possibly masquerade the existing problem with the
vector element. Try this

GroupSetTemplate& element =
invariantState.getGroupSetTemplates().at(0);
cout << "Gst name" << element.getName() << endl;

(note the use of reference) and see if it works fine or also throws a
segfault. This would be a better test, compared to your version with a copy.

Otherwise, the code you provided looks fine. The problem must be elsewhere.

--
Best regards,
Andrey Tarasevich

Salt_Peter

11/19/2008 3:10:00 AM

0

On Nov 18, 7:17 pm, Prasad <prasadmpa...@gmail.com> wrote:
> Hi,
>
> I have been using vector::iterators for a while now. This is the first
> time I have encountered this problem.
> The vector contains one element.
>
> 1. vector<GroupSetTemplate>::iterator gstIt;
> 2. for(gstIt= this->invariantState.getGroupSetTemplates().begin();
> gstIt!=this->invariantState.getGroupSetTemplates().end();gstIt++)
> 3{
> 4 cout<<"Gst name"<<(*gstIt).getName()<<endl;
> 5}
>
> Line 4 throws me segmentation fault.
>
> However if I rewrite the above code using vector::at() it works fine.
> GroupSetTemplate element = invariantState.getGroupSetTemplates().at
> (0);
> cout<<"Gst name"<<element.getName()<<endl;
>
> So I know that there is no problem with the element. Am I missing
> something ?

As already noted, at(0) returns a copy. Your original code involves
iterators. Lets bet member function getGroupSetTemplates() returns a
local variable (hence the seg fault).

The fact that you have a conditional expression in that for loop like
so:

gstIt!=this->invariantState.getGroupSetTemplates().end()

is indicative of poor programming practices.
You could always try a simplified reconstruction of your class
hierarchy and post a short, compilable snippet.

James Kanze

11/19/2008 9:33:00 AM

0

On Nov 19, 4:09 am, Salt_Peter <pj_h...@yahoo.com> wrote:
> On Nov 18, 7:17 pm, Prasad <prasadmpa...@gmail.com> wrote:
> > I have been using vector::iterators for a while now. This is
> > the first time I have encountered this problem.
> > The vector contains one element.

> > 1. vector<GroupSetTemplate>::iterator gstIt;
> > 2. for(gstIt= this->invariantState.getGroupSetTemplates().begin();
> > gstIt!=this->invariantState.getGroupSetTemplates().end();gstIt++)
> > 3{
> > 4 cout<<"Gst name"<<(*gstIt).getName()<<endl;
> > 5}

> > Line 4 throws me segmentation fault.

> > However if I rewrite the above code using vector::at() it works fine.
> > GroupSetTemplate element = invariantState.getGroupSetTemplates().at
> > (0);
> > cout<<"Gst name"<<element.getName()<<endl;

> > So I know that there is no problem with the element. Am I
> > missing something ?

> As already noted, at(0) returns a copy.

No. The function vector<>::at() returns a reference. He makes
the copy afterwards.

> Your original code involves iterators. Lets bet member
> function getGroupSetTemplates() returns a local variable
> (hence the seg fault).

Returns a local variable, or returns a reference to a local
variable. Either could certainly explain his symptoms. (So
could a few other things, but that sounds like a pretty good
guess.)

> The fact that you have a conditional expression in that for
> loop like so:

> gstIt!=this->invariantState.getGroupSetTemplates().end()

> is indicative of poor programming practices.

Why? I'd say it was more or less standard practice. In fact, a
for loop without a conditional expression would be an endless
loop. (It's not standard practice to consistently write out the
this->, of course, but maybe this code is in a template, and
invariantState is a member of a dependent base class.)

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

Prasad

11/20/2008 2:01:00 AM

0

On Nov 19, 3:33 am, James Kanze <james.ka...@gmail.com> wrote:
> On Nov 19, 4:09 am, Salt_Peter <pj_h...@yahoo.com> wrote:
>
>
>
> > On Nov 18, 7:17 pm, Prasad <prasadmpa...@gmail.com> wrote:
> > > I have been using vector::iterators for a while now. This is
> > > the first time I have encountered this problem.
> > > The vector contains one element.
> > > 1. vector<GroupSetTemplate>::iterator gstIt;
> > > 2. for(gstIt= this->invariantState.getGroupSetTemplates().begin();
> > > gstIt!=this->invariantState.getGroupSetTemplates().end();gstIt++)
> > > 3{
> > >  4            cout<<"Gst name"<<(*gstIt).getName()<<endl;
> > > 5}
> > > Line 4 throws me segmentation fault.
> > > However if I rewrite the above code using vector::at() it works fine.
> > > GroupSetTemplate element = invariantState.getGroupSetTemplates().at
> > > (0);
> > > cout<<"Gst name"<<element.getName()<<endl;
> > > So I know that there is no problem with the element. Am I
> > > missing something ?
> > As already noted, at(0) returns a copy.
>
> No. The function vector<>::at() returns a reference.  He makes
> the copy afterwards.
>
> > Your original code involves iterators. Lets bet member
> > function getGroupSetTemplates() returns a local variable
> > (hence the seg fault).
>
> Returns a local variable, or returns a reference to a local
> variable.  Either could certainly explain his symptoms.  (So
> could a few other things, but that sounds like a pretty good
> guess.)

getGroupSetTemplates() returns a class member. Is that a problem?


>
> > The fact that you have a conditional expression in that for
> > loop like so:
> > gstIt!=this->invariantState.getGroupSetTemplates().end()
> > is indicative of poor programming practices.
>
> Why?  I'd say it was more or less standard practice.  In fact, a
> for loop without a conditional expression would be an endless
> loop.  (It's not standard practice to consistently write out the
> this->, of course, but maybe this code is in a template, and
> invariantState is a member of a dependent base class.)
>
> --
> James Kanze (GABI Software)             email:james.ka...@gmail.com
> Conseils en informatique orientée objet/
>                    Beratung in objektorientierter Datenverarbeitung
> 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

Andrey Tarasevich

11/20/2008 2:07:00 AM

0

Prasad wrote:
> getGroupSetTemplates() returns a class member. Is that a problem?

What does "returns a class member" mean? Does it return a copy of a
class member? Or does it return a reference to a class member? It is a
problem if the former is true.

Did you make a test with 'at' and reference?

--
Best regards,
Andrey Tarasevich

Prasad

11/20/2008 2:12:00 AM

0

On Nov 19, 8:06 pm, Andrey Tarasevich <andreytarasev...@hotmail.com>
wrote:
> Prasad wrote:
> > getGroupSetTemplates() returns a class member. Is that a problem?
>
> What does "returns a class member" mean? Does it return a copy of a
> class member? Or does it return a reference to a class member? It is a
> problem if the former is true.
>
> Did you make a test with 'at' and reference?
>
> --
> Best regards,
> Andrey Tarasevich

Yes. The test with reference fails too. Any reason why the exact same
calls fail for reference but work for the copy?
Infact a valgrind analysis revealed a problem with string access,
which is in one of the innermost classes.

==32069== Process terminating with default action of signal 11
(SIGSEGV)
==32069== Access not within mapped region at address 0xFFFFFFFC
==32069== at 0x40CCD3A: std::string::string(std::string const&)
(in /usr/lib/libstdc++.so.6.0.9)

Any idea what this error means? AFAIK I am returning any local
references or variables.

~Prasad

Prasad

11/20/2008 2:52:00 AM

0

On Nov 19, 8:06 pm, Andrey Tarasevich <andreytarasev...@hotmail.com>
wrote:
> Prasad wrote:
> > getGroupSetTemplates() returns a class member. Is that a problem?
>
> What does "returns a class member" mean? Does it return a copy of a
> class member? Or does it return a reference to a class member? It is a
> problem if the former is true.

I was able to fix the problem.
//Original code
vector<GroupSetTemplates> getGroupSetTemplates()
{
return this->groupSetTemplates;
}

//Fixed code
vector<GroupSetTemplates>& getGroupSetTemplates()
{
return this->groupSetTemplates;
}

My understanding of this is
1. Initially a copy of groupSetTemplates was being returned. I was
getting an error because I was creating an iterator over returned
copy.
2. I am returning a reference of groupSetTemplates. A copy of the
reference is returned which still points to original groupSetTemplate
and hence my code works fine.

Is this understanding correct?
>
> Did you make a test with 'at' and reference?
>
> --
> Best regards,
> Andrey Tarasevich

Paavo Helde

11/20/2008 6:42:00 AM

0

Prasad <prasadmpatil@gmail.com> kirjutas:

> On Nov 19, 8:06 pm, Andrey Tarasevich <andreytarasev...@hotmail.com>
> wrote:
>> Prasad wrote:
>> > getGroupSetTemplates() returns a class member. Is that a problem?
>>
>> What does "returns a class member" mean? Does it return a copy of a
>> class member? Or does it return a reference to a class member? It is a
>> problem if the former is true.
>
> I was able to fix the problem.
> //Original code
> vector<GroupSetTemplates> getGroupSetTemplates()
> {
> return this->groupSetTemplates;
> }
>
> //Fixed code
> vector<GroupSetTemplates>& getGroupSetTemplates()
> {
> return this->groupSetTemplates;
> }
>
> My understanding of this is
> 1. Initially a copy of groupSetTemplates was being returned. I was
> getting an error because I was creating an iterator over returned
> copy.

The problem was that you returned two copies of groupSetTemplates, and
you were attempting to iterate from the beginning of the first to the end
of the second.

> 2. I am returning a reference of groupSetTemplates. A copy of the
> reference is returned which still points to original groupSetTemplate
> and hence my code works fine.
>
> Is this understanding correct?

Basically yes, this is what Andray has tried to talk you.

Terminological note: usually one does not talk about a "copy of a
reference" (though there might be some copying of bits involved in the
CPU level, but this is irrelevant). If the function returns a reference,
it just returns a reference.

Paavo

Prasad

11/20/2008 7:01:00 AM

0

On Nov 20, 12:41 am, Paavo Helde <pa...@nospam.please.org> wrote:
> Prasad <prasadmpa...@gmail.com> kirjutas:
>
>
>
> > On Nov 19, 8:06 pm, Andrey Tarasevich <andreytarasev...@hotmail.com>
> > wrote:
> >> Prasad wrote:
> >> > getGroupSetTemplates() returns a class member. Is that a problem?
>
> >> What does "returns a class member" mean? Does it return a copy of a
> >> class member? Or does it return a reference to a class member? It is a
> >> problem if the former is true.
>
> > I was able to fix the problem.
> > //Original code
> > vector<GroupSetTemplates> getGroupSetTemplates()
> > {
> > return this->groupSetTemplates;
> > }
>
> > //Fixed code
> > vector<GroupSetTemplates>& getGroupSetTemplates()
> > {
> > return this->groupSetTemplates;
> > }
>
> > My understanding of this is
> > 1. Initially a copy of groupSetTemplates was being returned. I was
> > getting an error because I was creating an iterator over returned
> > copy.
>
> The problem was that you returned two copies of groupSetTemplates, and
> you were attempting to iterate from the beginning of the first to the end
> of the second.
>
> > 2. I am returning a reference of groupSetTemplates. A copy of the
> > reference is returned which still points to original groupSetTemplate
> > and hence my code works fine.
>
> > Is this understanding correct?
>
> Basically yes, this is what Andray has tried to talk you.
>
> Terminological note: usually one does not talk about a "copy of a
> reference" (though there might be some copying of bits involved in the
> CPU level, but this is irrelevant). If the function returns a reference,
> it just returns a reference.
>
> Paavo

Thanks Paavo and Andray :)

Paavo Helde

11/20/2008 7:58:00 PM

0

Prasad <prasadmpatil@gmail.com> kirjutas:

> On Nov 20, 12:41 am, Paavo Helde <pa...@nospam.please.org> wrote:
>>
>> Basically yes, this is what Andray has tried to talk you.
>
> Thanks Paavo and Andray :)

Sorry Andrey, for misspelling your name (and having been copied!)

Paavo